International observers on July 19 will attempt to gain access to the crash site of a Malaysia Airlines passenger plane shot down in eastern Ukraine, after being hindered by armed pro-Russian rebels.

U.S. President Barack Obama has said evidence indicates the Malaysian airliner was hit by a surface-to-air missile launched from an area controlled by Russian-backed separatists.

The UN Security Council on July 18 unanimously called for a "full, thorough and independent international investigation" into the downing of the Malaysian airliner, which killed all 298 people on board.

The council urged all parties to grant investigators access to the site.

A group of monitors from the Organization for Security and Co-operation in Europe (OSCE) were prevented July 18 by separatists from fully inspecting the crash site.

The OSCE said about 30 observers and experts arrived by helicopter at the site after separatists announced they would allow international investigators into the area, which they control.

But OSCE spokesman Michael Bociurkiw said: "We encountered armed personnel who acted in a very impolite and unprofessional manner. Some of them even looked slightly intoxicated."

Thomas Greminger, the Swiss ambassador to the OSCE, whose country holds the organization's current chairmanship, said in Vienna that the monitors "did not have the freedom of movement they need."

Greminger said the observers would try again to gain access on July 19.

The separatists earlier claimed the two "black boxes" recording flight data had been found, but later denied those reports.

The plane was shot down July 17, killing all 298 people on board. It was flying from Amsterdam to Kuala Lumpur. It fell between Krasni Luch in Luhansk region and Shakhtarsk in the neighboring region of Donetsk.

Speaking in Washington July 18, Obama said Ukraine, Russia, and pro-Russian separatists must adhere to an immediate cease-fire and not interfere with international investigation into the causes of the downing of Malaysia Airlines flight MH17.

He said the "eyes of the world are on eastern Ukraine; we're going to make sure that truth gets out."

WATCH: U.S. President Barack Obama in his televised remarks from the White House on July 18 on the Malaysian airliner tragedy:

Obama said Russian President Vladimir Putin has the most control over the violent situation in Ukraine and must act to rein in the separatists, who "are heavily armed and trained because of Russian support."

The U.S. president said that time and time again Russia has refused to take concrete steps necessary to de-escalate the crisis in eastern Ukraine and Russia has continued to violate Ukrainian sovereignty and to support violent separatists.

Obama spoke separately on July 18 with German Chancellor Angela Merkel and British Prime Minister David Cameron about the potential need for more sanctions against Russia if it fails to take steps to de-escalate the situation in Ukraine.

Vice President Joe Biden, meanwhile, spoke for the second day running with Ukrainian President Petro Poroshenko and Polish Prime Minister Donald Tusk.

The White House said Biden and Poroshenko "agreed that as the party responsible for arming the separatists, Russia needed to publicly call on the separatists to lay down their weapons and grant immediate access to international and Ukrainian investigators."

Separately, Tusk and Biden noted that Russia was "supplying weapons and training -- including anti-aircraft weapons -- to the separatists, with profoundly de-stablilizing consequences."

The Ukrainian military and the separatists are blaming each other for shooting down the plane with a missile on July 17 in eastern Donetsk region.

U.S. and Ukrainian officials have suggested the plane was brought down with a Buk ground-to-air missile.

Both Ukrainian and Russian forces have SA-17 systems -- known as Buk launchers.

Authorities in Kyiv say they intercepted and recorded two telephone calls in which pro-Russian separatists were bragging about shooting down a plane in the area on July 17 before it became apparent that a civilian passenger plane had crashed there.

Passengers on Malaysia Airlines Flight MH17 included people from about a dozen nations. The crew members were all Malaysian.

A 62-member Malaysian disaster-response team including two air-accident investigators was due in Kyiv on July 19.

Malayisian Prime Minister Najib Razak told reporters late on July 18 that he spoke to Putin by phone to stress the need for an objective, unfettered probe into the crash. "I also told Putin that the site should not be tampered [with] before the team begins its investigation," he said.

The Malaysian government has confirmed that Razak's step-grandmother was on board the downed airliner.